1. Field of the Invention
This invention relates to engine retarders of the compression relief type. More particularly, the present invention relates to an hydraulic reset mechanism which insures that an exhaust valve (or valves) which was opened to produce the desired engine retarding effect is closed prior to the normal opening of the exhaust valve (or valves).
2. The Prior Art
Engine retarders of the compression relief type are well known in the art. Such retarders are designed to convert, temporarily, an internal combustion engine of the spark ignition or compression ignition type into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower normally developed by the engine.
The basic design for an engine retarding system of the type here involved is disclosed in the Cummins U.S. Pat. No. 3,220,392. In that design, an hydraulic system is employed wherein the motion of a master piston actuated by an appropriate intake, exhaust or injector pushrod or rocker arm controls the motion of a slave piston which opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke but, instead, is dissipated through the exhaust and cooling systems.
Various improvements have been made in the original design shown in the Cummins U.S. Pat. No. 3,220,392. Laas U.S. Pat. No. 3,405,699 discloses a device to unload the hydraulic system whenever excess motion of the slave piston tends to open the exhaust valve too far and hence risk damage to the components of the engine.
Sickler et al. U.S. Pat. No. 4,271,796 discloses a pressure relief system for a compression relief engine retarder wherein a bi-stable ball relief valve and a damping mechanism rapidly drops the pressure in the hydraulic system to a predetermined low level whenever an excess pressure is sensed in the hydraulic system, thereby obviating the risk of damage to various components in the engine valve train mechanism.
U.S. patent application Ser. No. 248,344, assigned to the assignee of the present invention, discloses an improved timing mechanism for an engine retarder which produces an increased retarding power while increasing the time span between the beginning of the engine retarding action and the beginning of the normal opening of the exhaust valves of the engine.
A further improvement in engine retarder operation is disclosed in U.S. patent application Ser. No. 124,581, assigned to the assignee of the present invention. Application Ser. No. 124,581 relates particularly to engines equipped with dual exhaust valves and an engine retarder of the compression relief type and discloses apparatus to open only one of the dual exhaust valves during retarder operation while permitting both valves to be opened during normal engine operation.
As has been set forth in the patents and applications referred to above, the compression relief engine retarder uses the existing engine valve train and fuel injector mechanisms to operate the exhaust valves. However, in the apparatus of the patents and application referred to above, the exhaust valve or valves opened by the retarder may still be opene when the normal opening of the exhaust valve is timed to commence. In this event, the rocker arm may impact sharply against the crosshead or valve stem and produce a loading condition which is different, and perhaps more severe, than that originally contemplated in the design of the engine. Such a loading condition may be particularly disadvantageous in the case of engines equipped with dual exhaust valves where the engine retarder is designed to act on only one valve. In this case, the original designed symmetrical loading of the crosshead and crosshead guide is transmuted into an asymmetrical loading condition whenever one of the exhaust valves is partially open when the second exhaust valve begins to open.